Such a rope is known from U.S. Pat. No. 5,901,632. In this patent publication a large-diameter braided rope is described, which rope contains a plurality of strands that themselves have been braided, preferably from rope yarns containing high-strength polymer fibres. In the most preferred embodiments indicated, the rope is a 12-strand, two-over/two-under circular braid, wherein each strand is itself a 12-strand braid made from high-performance polyethylene (HPPE) fibres (12×12 construction).
A rope for bend-over-sheave applications is within the context of the present application considered to be a load-bearing rope typically used in lifting and mooring applications; such as marine, oceanographic, offshore oil and gas, seismic, commercial fishing and other industrial markets. During such uses, together referred to as bend-over-sheave applications, the rope is frequently pulled over drums, bitts, pulleys, sheaves, etc., a.o. resulting in rubbing and bending. When exposed to such frequent bending or flexing, a rope may fail due to rope and fibre damage resulting from external and internal abrasion, frictional heat, etc.; such fatigue failure is often referred to as bending fatigue or flex fatigue,
To reduce flex fatigue of a rope in bend-over-sheave applications, use of a sheave (or other surface) with a diameter of at least 8 times the rope diameter is generally advised. In order to reduce loss of strength in a rope resulting from external abrasion, it is known to provide a jacket, for example a woven or braided sleeve, to the rope or to the strands in the rope. These jackets, however, increase rope diameter and stiffness, and add weight and cost, but do not contribute to the load bearing capacity of the rope; and direct visual inspection of the load bearing elements is not possible. In order to reduce a.o. loss of strength resulting from internal abrasion between the fibres in the rope, applying a specific mixture of polymer fibres in the rope strands is proposed in U.S. Pat. No. 6,945,153 B2.
The U.S. Pat. No. 6,945,153 B2 publication describes a braided rope of construction analogous to U.S. Pat. No. 5,901,632, wherein the strands contain a mixture of high-performance polyethylene fibres and lyotropic or thermotropic polymer fibres, in a ratio of 40:60 to 60:40. The lyotropic or thermotropic liquid crystalline fibres, like aromatic polyamides (aramids) or polybisoxazoles (PBO) are indicated to provide good resistance to creep rupture, but to be very susceptible to self-abrasion; whereas HPPE fibres are mentioned to exhibit the least amount of fibre-to-fibre abrasion, but to be prone to creep failure.
A drawback of known ropes, however, remains a limited service life when exposed to frequent bending or flexing. Accordingly, there is a need in industry for ropes that show improved performance in cyclic bend-over-sheave applications during prolonged times.
The object of the invention is therefore to provide such a rope showing improved performance.
This object is achieved according to the invention with a rope containing a plurality of strands comprising a mixture of high-performance polyethylene (HPPE) fibres and polytetrafluoroethylene (PTFE) fibres in a mass ratio of 70:30 to 98:2 for the rope in total.
Surprisingly a rope having an optimum of properties is obtained. The rope has an improved flex fatigue and yet still has a high stiffness and high strength.
The rope according to the invention shows markedly improved service life performance in cyclic bend-over-sheave applications, which is surprising because, although PTFE as such is known for among others its lubricating properties, in for example U.S. Pat. No. 6,945,153 B2 it is clearly stated that HPPE yarns would already show the best abrasion performance in ropes.
Other advantages of the rope according to the invention include that less heat, for example as a result of inter strand and/or inter fibre friction, is generated during use; which lowers the risk that the HPPE fibres show creep elongation. A rope comprising a high amount of HPPE fibres can thus be safely applied in long-term applications provided it is properly designed and used; for example by preventing overloading situations (versus maximum design capacity). The rope has high strength efficiency, meaning the strength of the rope is a relatively high percentage of the strength of its constituting fibres. The rope also shows good performance on traction and storage winches, and can be easily inspected on possible damage.
The present invention therefore also relates to the use of a rope of construction and composition as further detailed in this application as a load-bearing member in bend-over-sheave applications, for example in hoisting applications.